Field
The present disclosure generally relates to the fabrication of a hybrid optical source. More specifically, the present disclosure relates to fabrication of a hybrid optical source with two co-planar, edge-coupled optical devices coupled to a substrate.
Related Art
Optical signaling based on silicon photonics has the potential to alleviate off-chip bandwidth bottlenecks, as well as to provide low latency intra-chip and chip-to-chip communication. Optical interconnects with these capabilities can facilitate new system architectures that include multiple chips, with multi-threaded cores. These optical interconnects can provide: high-bandwidth, low-latency and energy-efficient data communication.
In the last few years, significant progress has been made in developing low-cost components for use in inter-chip and intra-chip silicon-photonic links, including: high-bandwidth efficient silicon modulators, low-loss optical waveguides, wavelength-division-multiplexing (WDM) components, and high-speed CMOS optical-waveguide photodetectors. However, producing a low-cost, efficient optical source (such as a laser) on silicon remains a challenge and poses an obstacle to implementing silicon-photonic links.
In particular, because of its indirect band-gap structure, silicon is a poor material for light emission and, therefore, usually has a low optical gain. In spite of ongoing efforts to enhance the light-emitting efficiency and optical gain of silicon, electrically pumped room-temperature continuous-wave (CW) lasing remains an elusive goal. Recent research efforts have included attempts to use germanium directly grown on silicon as a gain medium. However, high tensile strain and high doping are typically used to make germanium direct band gap, which can significantly reduce the wall-plug efficiency (WPE) of a resulting optical source.
Another approach for building lasers on silicon is to use III-V compound-semiconductor materials on silicon for efficient light emission. However, epitaxial growth of III-V compound semiconductors on silicon is typically difficult because of the large lattice and thermal mismatch between silicon and the III-V compound semiconductors, which often severely limits the laser efficiency and reliability. One solution to these challenges is hybrid wafer integration of III-V compound semiconductors with silicon. For example, evanescent-coupled hybrid lasers have been successfully demonstrated using wafer bonding of indium-phosphide optical devices to silicon via either oxide-to-oxide fusion bonding or benzocyclobutene bonding. Nonetheless, the WPE of these hybrid optical sources has been limited to around 5% because of taper loss, carrier-injection efficiency and thermal impedance. In addition, new fabrication techniques are needed to allow bonding of III-V compound semiconductors with active silicon devices that include multiple layers of metal interconnects and interlayer dielectrics.
Other proposed hybrid optical sources employ surface-normal coupling of a III-V compound-semiconductor gain medium with optical waveguides on silicon. For example, a surface-normal optical coupler on a III-V compound-semiconductor can be implemented using an etched optical waveguide facet, and a surface-normal optical coupler on silicon can be implemented using grating optical couplers. In principle, with an optical mode field diameter or optical mode sizes of 5 μm, grating optical couplers with an insertion loss of around −1 dB can be achieved, allowing efficient optical coupling of light into and out of a submicron, silicon optical waveguide. However, such a large optical mode field diameter is typically not available for III-V compound-semiconductor optical waveguide designs, which usually have optical modes sizes of around 1 μm. At these optical mode sizes, the optical coupling loss of an etched optical waveguide facet or mirror with perfect alignment is larger than 3 dB.
Alternatively, proposed hybrid optical sources employing edge-to-edge butt-coupling of a III-V compound-semiconductor gain medium with silicon optical waveguides can provide similar electrical injection efficiency and thermal impedance as conventional III-V compound-semiconductor lasers. For example, external cavity lasers using this hybrid integration technique have been successfully demonstrated with a WPE of up to 9.5%. However, because of optical mode mismatches between the III-V compound-semiconductor and silicon optical waveguides, optical mode spot-size converters on either or both of the III-V compound-semiconductor optical device and the silicon-based optical device are typically needed. In addition, accurate alignment (with submicron alignment tolerances) is usually needed for efficient optical coupling. Achieving such six-axis alignment in a high-yield and low-cost fabrication technique has proven to be very difficult.
Hence, what is needed is a technique for fabricating a hybrid optical source without the problems described above.